1. Field of the Invention
The present invention relates to a ceramic laminated circuit substrate suitable for functional modules, a method for making the same and use of the substrate.
2. DESCRIPTION OF RELATED ART
Recently, high density integration has been demanded for increasing operation speed. For LSI chips, fine wiring has become possible owing to progress of wiring techniques and pitches of terminals for input and output of electric signal from LSI chips become fine, namely, less than 500 .mu.m.
Further, with reference to integration, high density wiring has become possible by employing ceramic laminated circuit substrates. Connection of LSI chip and ceramic laminated circuit substrate is now carried out by mounting LSI chip directly or through the medium of a carrier substrate on ceramic laminated circuit substrate for high density integration.
However, in spite of the fact that LSI chips are becoming finer because of progress in technique, ceramic laminated circuit substrate is made by sintering to cause shrinkage of 10-30% and as a result there occurs the problem that connection between the LSI chip and the ceramic laminated circuit substrate cannot satisfactorily be attained due to uneven shrinkage. The unevenness or fluctuation in shrinkage occurs because degree of shrinkage is relatively high, namely, 10-30% and besides this degree of shrinkage cannot be accurately controlled. In general, fluctuation in shrinkage is closely connected with degree of shrinkage and with increase in degree of shrinkage, fluctuation increases.
In order to decrease fluctuation in degree of shrinkage, it is necessary to produce a molded product which is uniform and less in unevenness in density by controlling particle size of raw material powder and distribution of particle size or selecting dispersing method, binders and press conditions. However, even with such control of conditions, fluctuation in shrinkage at firing is at best about .+-.0.1-0.2% and this cannot cope with enhancement in density of wiring.
It is known to improve performances of ceramics by incorporating fibers into ceramics to form a composite ceramic. For example, it is known to improve toughness by incorporating fibers into ceramics. Furthermore, Japanese Patent Kokai (Laid-Open) No. 59-83985 discloses a lightweight and highly heat insulating ceramic substrate obtained by incorporating a foaming agent and fibers in glass. Moreover, Japanese Patent Kokoku (Publication) No. 51-16302 and Japanese Patent Kokai (Laid-Open) No. 55-117250 disclose conductors comprising a composite of carbon fibers and copper conductor. In addition, an organic multilayer circuit substrate is known which is mainly improved in strength and lowered in thermal expansion coefficient and which is prepared by incorporating glass cloth in an organic material. However, this is still not sufficiently low in the thermal expansion coefficient and has problem in reliability in connecting it to LSI chip. Furthermore, as disclosed in Japanese Patent Kokai (Laid-Open) No. 62-270442, composite ceramic materials are obtained by incorporating whiskers in a glass, but such materials cannot be sintered densely under atmospheric pressure and voids may remain in the products.
These composite ceramics are for improvement of toughness, strength and heat insulating properties and for lightening in weight and not for improvement of accuracy in size. For improvement of accuracy in size, Japanese Patent Kokai (Laid-Open) No. 62-241391 discloses that when a glass cloth comprising glass fibers is embedded in a green sheet for ceramic wiring substrates, generation of strain is inhibited owing to reinforcing action of the glass cloth and accuracy in size of each layer at lamination is improved.
The conventional ceramic laminated circuit substrates which are made by laminating and firing only the green sheets containing no fibers have the defects that degree of shrinkage at firing in the direction of laminate face is great and besides fluctuation thereof is great. When only the composite green sheets containing glass cloth are laminated and fired, since these green sheets shrink with difficulty in the direction of thickness, 15-30 vol% of voids are apt to be generated in the sheets.